1. Field of the Invention
The present invention relates to a Raman amplifier for amplifying signal light for an optical communication, an optical communication system comprising the Raman amplifier, and a controlling device of the Raman amplifier.
2. Description of the Related Art
Raman amplifiers are being put into practical use as a technique for building a network for a long-distance transmission optical communication system that can perform large-capacity communications. The Raman amplifier uses an optical fiber as an amplification medium by making pump light having a high intensity be incident to a transmission line fiber.
The Raman amplifier uses the physical phenomenon whereby a Raman amplification effect occurs in the wavelength range that depends on the wavelength of pump light, as shown in FIG. 1, as a result of making the pump light having a certain wavelength be incident to an optical fiber. In the example shown in FIG. 1, Raman gains 111 to 113 are respectively generated by pump lights 101 to 103 having different wavelengths. For quartz glass used as an optical fiber, its maximum amplification characteristic exists in the wavelength range of a frequency that is lower than the wavelength of pump light by approximately 13.2 THz. Accordingly, to Raman-amplify, for example, signal light in the vicinity of 1550 nm, a Raman gain can be efficiently obtained if pump light having a wavelength in the vicinity of 1450 nm is used.
In an optical communication system in which signal light of a broad wavelength range must be collectively amplified, as in a WDM (Wavelength Division Multiplexing) transmission, the amplification characteristic of a broad wavelength range according to the intensities and the wavelengths of pump lights can be obtained by using a plurality of pump lights having mutually different wavelengths, and by respectively controlling the intensities of the pump lights.
To control a desired amplification characteristic, the Raman amplifier normally has the ability to adjust the intensity of pump light so that a predetermined gain can be obtained while monitoring the intensity of signal light. Meanwhile, amplified spontaneous scattering (ASS) light occurs within a fiber with the Raman amplification effect within the optical fiber.
This ASS light occurs in the same direction as the transmission direction and in the same wavelength range as the signal light. Therefore, the ASS light mixes as a noise component along with the signal light when the intensity of the signal light is monitored. Accordingly, the Raman amplifier has the ability to detect the intensity of a signal light by subtracting the ASS light generation amount, which is a noise component, in order to obtain a predetermined signal light intensity.
As a technique for obtaining information about the ASS light generation amount, there is a method for deriving a relational expression between the intensity of pump light and the ASS light generation amount for an optical fiber having a certain optical characteristic, and for calculating the ASS light generation amount from a monitored intensity of pump light pursuant to the relational expression, since it is known that the ASS light generation amount has a correlation with the intensity of pump light made incident to an optical fiber.
For example, a Raman amplifier is provided with the ability to monitor the intensity of pump light made incident to an optical fiber, and a relational expression between the intensity of pump light and the ASS light generation amount is stored in a storage element within the Raman amplifier and used for a computation process, whereby the ASS light generation amount can be calculated from the monitored intensity of pump light.
However, the following problems must be overcome to realize an optical communication system comprising a Raman amplifier for implementing a long-distance transmission.    (1) Since a Raman amplifier uses an optical fiber, which is a transmission line, as an amplification medium, individual differences occur in the optical characteristics of transmission line fibers due to a) local optical loss which occurs at a site where the Raman amplifier is placed, such as loss in the connecting portion of an optical fiber connected to the Raman amplifier, bending loss, etc.; b) the manufacturing process of an optical fiber; and c) the elapsed time/temperature environment.
If pump light having the same intensity is made incident to optical fibers the optical characteristics of which differ due to the above described factors a) to c), then the degree of Raman amplification resultant from the Raman scattering effect according to the optical characteristic of a transmission line fiber and the ASS light generation amount that occurs as a noise component with the Raman amplification are different. As a result, with the conventional method for estimating the ASS light generation amount from the intensity of pump light, the accuracy of estimation deteriorates, leading to difficulties in the accurate calculation of the intensity of signal light from which the ASS light generation amount is subtracted.
In a long-distance transmission, the intensity of signal light must be calculated/monitored by correcting ASS light as a noise component, and a satisfactory transmission characteristic must be obtained in each optical amplifier that configures an optical communication system. Accordingly, improving the accuracy of estimation of the ASS light generation amount in a Raman amplifier is necessary to realizing improved long-distance transmission characteristics.    (2) If an optical communication system encounters an abnormal condition due to a cause such as a fault in an upstream station, the disconnection of a transmission line, etc., the abnormality must be detected and the system must be automatically shut down. The Raman amplifier has the ability to detect the presence/absence of signal light by monitoring the intensity of the signal light in order to detect the above described abnormality. When this detection capability detects that the signal light does not reach, it transmits a message that communications are abnormal, and shuts down the system.
However, since ASS light mixes along with the signal light as a result of the Raman amplification, the accuracy of detecting the presence/absence of signal light becomes problematic in some cases. In particular, in a WDM transmission using a broad wavelength range, the number of mixed noise components becomes large. Therefore, it is sometimes undetectable that a signal light is not reaching if the accuracy of estimation of ASS light is low. This poses a problem from the viewpoint of security of an optical communication system.
The following methods for estimating/correcting the ASS light generation amount are known as conventional techniques that overcome these problems.
(1) Patent Document 1
The loss distribution of an optical fiber to which a Raman amplifier is connected is measured with a measurement instrument such as an optical time domain reflectometer, etc., prior to the placement of the Raman amplifier. Additionally, the ASS light generation amount, which is measured beforehand in accordance with the intensity of pump light, and the characteristic of the optical fiber is stored in a storage element within the amplifier. Then, the ASS light generation amount in accordance with the individual differences of the optical characteristics of the optical fibers is estimated by inputting the optical characteristic of the optical fiber, which is obtained by measurement, into the Raman amplifier as input information when the Raman amplifier is set up, and by extracting and using information suitable for the characteristic of the optical fiber from the storage element.
(2) Patent Document 2
Pump light is made to be incident from a Raman amplifier to a transmission line in a state in which signal light is intercepted when the Raman amplifier is set up, and a correlation between the intensity of pump light and the ASS light generation amount of a connected optical fiber is measured. Then, the current ASS light generation amount is estimated from the ASS light generation amount that was measured when the Raman amplifier was set up in accordance with the monitored intensity of pump light when the optical communication system is operated. Otherwise, the accuracy of estimation of the ASS light generation amount is improved by applying a correction based on the ASS light generation amount that is measured at the set-up time to a prepared calculation expression of the ASS light generation amount.
However, the following problems still remain in the above described methods (1) and (2).
With the above described method (1), the length of time taken to make the measurement beforehand in the design phase of the Raman amplifier and the amount of information stored in the Raman amplifier increase as the individual differences in the optical characteristics of an optical fiber connected to the Raman amplifier become large, leading to inefficiencies. Furthermore, an error in the estimation of the ASS light generation amount caused by a mismatch between optical characteristics occurs if the optical characteristic of the connected optical fiber and that of an optical fiber that is measured beforehand or stored do not match and information about an optical fiber having a similar optical characteristic is used.
With the above described method (2), the ASS light generation amount must be measured under a condition in which signal light does not pass through. Therefore, procedures for preparing system operations become complicated. Furthermore, if the intensity of pump light at the time of system operations and that of pump light that is measured when the Raman amplifier is set up do not match (if the ratios of the intensities of pump lights do not match when the wavelengths of the plurality of pump lights are used), then the accuracy of estimation of the ASS light generation amount deteriorates.
In addition, with the above described methods (1) and (2), a correction is made on the basis of measurement information obtained in the design phase of the system or prior to the operation of the system. Therefore, if the optical characteristic of an optical fiber varies with a secular/environmental change, then the degree of Raman amplification and the ASS light generation amount will also vary at the same time. However, a change in the fiber characteristic is not considered when the ASS light generation amount, which occurs with the Raman amplification, is estimated from the intensity of pump light made incident to a transmission line fiber.
Accordingly, a correlation between a Raman gain, the ASS light generation amount, and the intensity of pump light varies in an optical fiber in which the optical characteristic varies with long term system operations. Therefore, estimation of the ASS light generation amount at a high level of accuracy cannot be guaranteed. This leads to a problem in which the accuracy of estimation deteriorates because a calculation formula for accurately estimating the ASS light generation amount cannot be corrected to cope with a change in the ASS light generation amount that occurs with a change in the optical characteristic of the optical fiber.
Furthermore, for example, the method recited in Patent Document 3 is known as a conventional technique for monitoring the state of a transmission line fiber. With this method, the ability to monitor the intensity of pump light that proceeds in reverse to signal light within a transmission line fiber is comprised to monitor faults such as the disconnection of a transmission line fiber, the opening of an optical connecting portion, and the like. However, the ability to monitor a change in the optical characteristic of the optical fiber that is caused by a secular/environmental change is not comprised.    Patent Document 1: Japanese Published Unexamined Patent Application No. 2002-296145    Patent Document 2: Japanese Published Unexamined Patent Application No. 2004-287307    Patent Document 3: Japanese Published Unexamined Patent Application No. 2004-172750
As described above, the optical characteristic of a transmission line of an optical transmission system using a Raman amplifier normally varies by system. For this reason, an obtained Raman gain and the intensity of ASS light that occurs as a noise component vary in accordance with the optical characteristic of a transmission line, which serves as an amplification medium, even if pump light having the same intensity is made incident to the transmission line fiber.
For example, if a Raman gain and the ASS light generation amount are calculated only from information on the intensities of the pump light and signal light, which are monitored by a Raman amplifier, as in a conventional technique, information on the transmission line will be lacking. Therefore, the pump light is controlled on the basis of transmission line information on the optical characteristic of a representative optical fiber used as a reference.
Because the optical characteristic of a transmission line to which a Raman amplifier is connected is different from that of the optical fiber that is used as a reference, in most cases the ASS light generation amount and the Raman gain in an actual transmission line are different from those estimated by the Raman amplifier. Accordingly, an accurate ASS light generation amount cannot be used when the ASS light generation amount is subtracted from the total intensity of light including signal light and the ASS light. As a result, the intensity of the signal light cannot be accurately grasped.
Patent Document 1 recites the method for measuring the optical characteristic of a transmission line beforehand and for compensating for a mismatch between transmission line characteristics by correcting the estimation error of the ASS light generation amount; this method is recited as a solution to a problem in which the ASS light generation amount cannot be accurately calculated due to a lack of information about the transmission line connected to a Raman amplifier. The measurement of an optical characteristic is an operation performed at the time of the setting up of a Raman amplifier during the preparatory phase before operations are begun. Therefore, suitable corrections are not made to changes in the optical characteristic of a transmission line fiber that occur during operations.